This application claims priority from EP01301283.6 filed Feb. 13, 2001, herein incorporated by reference.
1. Field of the Invention
The present invention relates generally to lithographic projection apparatus and more particularly to lithographic projection apparatus including an interferometric measurement system.
2. Background of the Related Art
A typical lithographic projection apparatus includes:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam, according to a desired pattern, in an object plane traversed by the projection beam;
a substrate table for holding a substrate;
a projection system downstream of said object plane, for projecting the patterned beam onto a target portion of the substrate;
an interferometric measurement system for measuring wave front aberrations of the projection system.
The term xe2x80x9cpatterning structurexe2x80x9d as here employed should be broadly interpreted as referring to structure that can be used to endow an incoming radiation beam with a patterned cross-section in an object plane traversed by the projection beam, corresponding to a pattern that is to be created in a target portion of the substrate. Said target portion is, through the projection system, optically conjugate to the object plane. The projection system has a magnification factor M (generally  less than 1) in relation to said object plane. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:
A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning structure can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (with M  less than 1) the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. The position of a second element traversed by the projection beam relative to a first element traversed by the projection beam will for simplicity hereinafter be referred to as xe2x80x9cdownstreamxe2x80x9d of or xe2x80x9cupstreamxe2x80x9d of said first element. In this context, the expression xe2x80x9cdownstreamxe2x80x9d indicates that a displacement from the first element to the second element is a displacement along the direction of propagation of the projection beam; similarly, xe2x80x9cupstreamxe2x80x9d indicates that a displacement from the first element to the second element is a displacement opposite to the direction of propagation of the projection beam. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
There is a desire to integrate an ever-increasing number of electronic components in an IC. To realize this it is necessary to decrease the size of the components and therefore to increase the resolution of the projection system, so that increasingly smaller details, or line widths, can be projected on a target portion of the substrate. For the projection system this means that the projection system and the lens elements used in the projection system must comply with very stringent quality requirements. Despite the great care taken during the manufacturing of lens elements and the projection system they both may still suffer from wave front aberrations, such as, for example, displacement, defocus, astigmatism, coma and spherical aberration across an image field projected with the projection system onto a target portion of the substrate. Said aberrations are important sources of variations of the imaged line widths occurring across the image field. It is important that the imaged line widths at different points within the image field are constant. If the line width variation is large, the substrate on which the image field is projected may be rejected during a quality inspection of the substrate. Using techniques such as phase-shifting masks, or off-axis illumination, the influence of wave front aberrations on the imaged line widths may further increase.
During manufacture of a lens element it is advantageous to measure the wave front aberrations of the lens element and to use the measured results to tune the aberrations in this element or even to reject this element if the quality is not sufficient. When lens elements are put together to form the projection system it may again be necessary to measure the wave front aberrations of the projection system. These measurements may be used to adjust the position of certain lens elements in the projection system in order to minimize wave front aberrations of the projection system.
After the projection system has been built into a lithographic projection apparatus, the wave front aberrations may be measured again. Moreover, since wave front aberrations are variable in time in a projection system, for instance, due to deterioration of the lens material or lens heating effects local heating of the lens material), it may be necessary to measure the aberrations at certain instants in time during operation of the apparatus and to adjust certain movable lens elements accordingly to minimize wave front aberrations. The short time scale, on which lens-heating effects may occur, may require measuring the wave front aberrations frequently.
The use of an interferometric measurement system for in-situ measurement of wave front aberrations of the projection system of a lithographic projection apparatus is described in P. Venkataraman, et al., xe2x80x9cAberrations of steppers using Phase Shifting Point Diffraction Interferometryxe2x80x9d, in Optical Microlithography XIII, J. Progler, Editor, Proceedings of SPIE Vol. 4000, 1245-1249 (2000). A Phase Shifting Point Diffraction Interferometry method and a corresponding system are disclosed in P. P. Naulleau et al., U.S. Pat. No. 6,100,978, issued Aug. 8, 2000, incorporated herein by reference. The Phase Shifting Point Diffraction Interferometry measurement method and corresponding measurement system will be referred to hereinafter as the PSPDI method and PSPDI system, respectively. The disclosed PSPDI systems feature the following elements, mentioned here in the order wherein these elements are traversed by the projection beam: a first pinhole in an object plane; a grating (with a one dimensional periodic structure of lines and spaces) between the object plane and the projection system, for generating by diffraction a test beam and a reference beam; the projection system, and a set of two pinholes comprising a window pinhole (traversed by the test beam) and a reference pinhole (traversed by the reference beam, and acting as a spatial filter for generating an unaberrated reference beam) in the plane that is optically conjugate to the object plane. The test beam and the reference beam generate an interference fringe pattern on a detector surface downstream of the set of two pinholes. This interference fringe pattern carries information on wave front aberrations. The grating, generally embodied as a grating pattern on a plane surface of a carrier substrate, acts as a beamsplitter; the grating shall be located downstream of said object plane such as to provide sufficient lateral separation of the areas traversed by the reference beam and the test beam in the plane that is optically conjugate to the object plane. Further, the grating is movable in a direction perpendicular to the direction of propagation of the projection beam such as to provide xe2x80x9cphase shiftingxe2x80x9d (as explained below) of the interference fringe pattern with respect to a coordinate system associated with the detector surface, as needed for measuring aberrations.
Said phase shifting of the interference fringe pattern involves shifting the interference fringe pattern with respect to said coordinate system. For an explanation of xe2x80x9cphase shiftingxe2x80x9d in relation to interferometry see, for example, D. Malacara, xe2x80x9cOptical Shop Testingxe2x80x9d, John Wiley and Sons, Inc., New York, second edition. Movement of an optical element (such as, for example, a grating) to provide phase shifting will be referred to hereinafter as xe2x80x9cphase steppingxe2x80x9d. A finite movement of an optical element (such as, for example, a grating) to provide a finite phase shift of said interference fringe pattern will be referred to hereinafter as a xe2x80x9cphase stepxe2x80x9d.
An embodiment of a PSPDI system in a lithographic projection apparatus comprises, besides the support structure for supporting patterning structure and the substrate table for holding a substrate, one or more dedicated, movable support structures for supporting the grating and/or for moving the grating into and out of the projection beam and/or for phase stepping the grating. Incorporation of these one or more dedicated support structures into the lithographic projection apparatus leads to added mechanical complexity and increased costs of manufacturing the lithographic projection apparatus. Further, as explained above, in a PSPDI system each individual beam (the test beam and the reference beam) impinging on the detector has traversed two pinholes, one pinhole upstream of the projection system, and one pinhole downstream of the projection system. This circumstance typical for a PSPDI system poses a limitation to the amount of radiation that may reach the detector, and hence, to the sensitivity of the measurement system.
One aspect of embodiments of the present invention provides a measurement system for measuring the wave front aberrations in a lithographic projection apparatus while alleviating the problem of incorporating one or more dedicated, movable support structures.
This and other aspects are achieved according to the invention in a lithographic projection apparatus as specified in the opening paragraph, characterized in that the interferometric measurement system comprises
a grating, featuring a grating pattern in a grating plane, said grating being movable into and out of the projection beam, such that the grating plane is substantially coincident with said object plane;
a pinhole, featuring a pinhole pattern in a pinhole plane and arranged in a pinhole plate, said pinhole being movable into and out of the projection beam, such that the pinhole plane is substantially coincident with a plane downstream of the projection system and optically conjugate to said object plane, and
a detector with a detector surface substantially coincident with a detection plane, said detection plane located downstream of the pinhole at a location where a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier Transformation of a spatial distribution of the electric field amplitude of the projection beam in the pinhole plane.
With the measurement system built into the lithographic projection apparatus it is possible to measure in situ the wave front aberration of the projection system. The term xe2x80x9cgrating patternxe2x80x9d in the context of the present invention should be interpreted throughout this text and in the claims to include any periodic structure. Also the term xe2x80x9cpinhole patternxe2x80x9d in the context of the present invention should be interpreted throughout this text and in the claims to include one or more apertures of arbitrary shape such as, for example a circular shape, a slit shape, a rectangular shape, and a substantially square shape. Upon illumination of the grating, an intensity distribution featuring an interference fringe pattern of radiation representative of the wave front aberration of the projection system is obtained in said detection plane downstream of the pinhole. In the absence of any wave front aberration said interference fringe pattern is a substantially uniform intensity distribution. In the presence of wave front aberrations said intensity distribution will be non-uniform, and will generally comprise interference fringes. In the context of the present invention, the term xe2x80x9cinterference fringe patternxe2x80x9d besides referring to the common concept of interference fringes, should also be interpreted as referring to a substantially uniform intensity distribution, the latter intensity distribution being typical for the absence of aberrations. Some of the physical principles exploited in the present invention are discussed, for example, in J. Braat et al., xe2x80x9cImproved Ronchi test with extended sourcexe2x80x9d, Journal of the Optical Society of America, Vol. 16, 131-139 (1999). Said detection plane may, for example, be located at a position downstream of the pinhole where the xe2x80x9cFraunhofer Diffractionxe2x80x9d approximation is applicable to the calculation of the electric field amplitude of radiation emerging from the pinhole. At such a location a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier Transformation of a spatial distribution of the electric field amplitude of the projection beam in the pinhole plane.
Phase shifting of the interference fringe pattern with respect to a coordinate system associated with the detector surface, as needed for measuring aberrations, can be provided by phase stepping either the grating or the pinhole.
In case the patterning structure is a mask and the support structure is a mask table for holding the mask, the grating can be provided to a grating module that has the same outer extent as the mask, and the mask table can be used for holding the grating module. During measurement of the wave front aberrations the grating module may be held at a location where, during normal use of the projection apparatus, a mask is held. One advantage of this scenario is that there is no need to provide an additional support structure to hold the grating, and/or to move the grating in and out of the projection beam, and/or to phase step the grating. Another advantage is that, during projection, the mass of the grating is not added to the mass of the mask table, such that it is easier to accelerate and decelerate the mask table. As explained above, in a PSPDI system each individual beam (the test beam and/or the reference beam) impinging on the detector has traversed two pinholes, one pinhole upstream of the projection system, and one pinhole downstream of the projection system. These two pinholes are each embodied as a single pinhole aperture, and in contrast to the present invention do not feature a pinhole pattern that may comprise a plurality of apertures. This circumstance, typical for a PSPDI system, poses a limitation to the amount of radiation that may reach the detector. Another advantage of the present invention over the use of a PSPDI system is that said limitation can be relaxed by the use of pinhole patterns comprising a plurality of apertures, leading to improved sensitivity.
Instead of providing the grating to a grating module that has the same outer extent as the mask, one can, alternatively, provide the grating to the support structure at a location away from the location where the patterning structure is supported. Whenever it is necessary to measure the wave front aberrations of the projection system the grating can be easily moved into the projection beam to perform a wave front aberration measurement. After finishing the measurement the patterning structure is moved into the projection beam and the apparatus can continue projecting the patterned beam onto target portions of the substrate. This method of intermittently measuring wave front aberrations during operation of the lithographic projection apparatus is time saving, and enables, for example, a frequent measuring of wave front aberrations needed to compensate for short-time-scale lens-heating effects. In an alternative scenario, the patterning structure may also be used to pattern the projection beam with a grating pattern in its cross section. This is advantageous because no additional grating has to be provided to the apparatus.
The detector for detecting the radiation traversing the pinhole may, for example, be provided to the substrate table. Said pinhole plate may also, for example, be provided to the substrate table. One could also provide the pinhole plate and the detector to a sensor module which, during measurement of wave front aberrations, may be provided to the substrate table at a location where, during the projection of the patterned beam, the substrate is held. After measuring wave front aberrations of the projection system the sensor module may then be replaced by the substrate, such that the projection of the patterned beam onto the target portions of the substrate may continue. An advantage is that when the sensor and the pinhole plate are built into a sensor module, the mass of the sensor module will not add to the mass of the substrate table during normal operation of the lithographic projection apparatus, and a further advantage is that the sensor doesn""t occupy any space in the substrate table.
According to a further aspect of the present invention, there is provided a method of measuring wave front aberrations of a projection system in a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a support structure for supporting patterning structure, the patterning structure serving to pattern the projection beam, according to a desired pattern, in an object plane traversed by the projection beam;
a substrate table for holding a substrate;
a projection system downstream of said object plane, for projecting the patterned beam onto a target portion of the substrate;
an interferometric measurement system for measuring wave front aberrations of the projection system,
characterized in that the method comprises the following steps:
providing a grating, featuring a grating pattern in a grating plane, into the projection beam, such that the grating plane is substantially coincident with said object plane;
providing a pinhole and a detector to the projection beam at a location downstream of the projection system, such that radiation traversing the pinhole is detectable by the detector, whereby said pinhole is arranged in a pinhole plate and features a pinhole pattern in a pinhole plane, the pinhole plane being substantially coincident with a plane that is optically conjugate to said object plane, and whereby said detector comprises a detector surface that is substantially coincident with a detection plane downstream of the pinhole, whereby, in said detection plane, a spatial distribution of the electric field amplitude of the projection beam is substantially a Fourier Transformation of a spatial distribution of the electric field amplitude in the pinhole plane;
illuminating the grating with the projection beam of radiation, and
detecting an interference fringe pattern of radiation with said detector.
By phase stepping the grating and/or the pinhole the interference fringe pattern will move over the detector surface. Intensities, at a plurality of points on the detector surface, and detected as a function of phase step, can be used for calculating the wave front aberration, as is discussed, for example, in D. Malacara, xe2x80x9cOptical Shop Testingxe2x80x9d, John Wiley and Sons, Inc., New York, second edition, chapter 14. This measurement has to be repeated at a particular measurement position a plurality of times, whereby the grating or the pinhole has to be moved over a distance equal to a phase step in one or more preselected directions along which the grating pattern or the pinhole pattern is periodic. A phase step of, for example, the grating should preferably be smaller or equal to ⅓ of the period (or ⅓ of the grating period plus an integer number of grating periods) of the grating pattern along said one or more preselected directions. The term period refers to the distance over which a periodic structure along a preselected direction in the grating pattern is repeated. With this measurement one can measure wave front aberrations at a particular measurement position and in a particular direction in the imaged field. To obtain information on wave front aberrations at a plurality of points in the field one should measure the wave front aberration at a corresponding plurality of measurement points in the imaged field along at least two directions. Phase stepping can be provided by moving the grating and/or moving the pinhole. It is advantageous to move the grating because, due to the magnification M (with M less than 1) of the projection system, the accuracy requirements for moving the grating (located upstream of the projection system) by the mask table are lower than for moving the pinhole (located downstream of the projection system).
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).